Smaller to medium sized engines for industrial vehicles, such as counter-balanced, rider, lift trucks, in the 2-6 thousand pound (900-2,700 kg) capacity range run at high rpm's to deliver the required power range and torque output demanded. Many industrial applications that involve lift trucks are in high volume warehouse operations. The trucks are largely confined to aisles or traffic lanes and work in close proximity to warehouse employees. This requires special attention to environmental and human factors. The noise level, as well as the emissions, from the many truck engines is a significant consideration. Despite quieter trucks, the trend is away from gasoline or diesel fuel systems and toward so-called cleaner burning fuels, such as liquid petroleum gas, or propane and methane because of air quality environmental regulations. For example, a fuel system for lift trucks of this type is described in a co-pending application entitled Alternative Fuel System for Powered Industrial Vehicles, Ser. No. 07/860,536, filed Mar. 30, 1992, assigned to the assignee of the present invention.
Unlike a gasoline or diesel fuel system where the fuel reservoir or tank is filled with liquid fuel at atmospheric pressure, and a fuel pump delivers the fuel to a carburetor or to injectors to be atomized and mixed with air in the proper proportions for delivery to the intake manifold of the engine, in a gaseous induction fuel system, fuel at several atmospheres of pressure, e.g. about 180 psi, is delivered to a regulator, the pressure is reduced in the regulator body and gaseous fuel is drawn by the negative manifold pressure to a mixer for combining with air in the proportions required for combustion in the engine. The fuel is contained in a bottle which is removed from the truck when empty, stored in racks outside the building and exchanged for a full bottle recharged with the cubic feet of gas to run the truck for a typical eight-hour work shift.
One of the difficulties with engines that idle at high rpm's and have the type of induction fuel system described is that the inertial effect of the flywheel continues to crank the engine after the ignition is shut off. This causes the manifold vacuum to persist momentarily which is sensed by the induction system as an engine running condition, although the ignition key has been turned off. The overrun is of short duration, yet it results in fuel mixture being drawn into the combustion chambers of the engine and if the engine is restarted before this fuel has dissipated, a pre-ignition may result, possibly aggravated by a slight timing misadjustment, and the engine backfires. Even if the truck meets or exceeds OSHA noise levels in normal operation, this condition is sure to be objectionable in a work place where many trucks are working.
While one option may be to leave the engine running during brief interruptions of operation, today's air quality standards require so-called clean air in fully ventilated buildings, and these air standards could not be met if trucks were left running when not in use